The present invention relates to a method for regenerating tetraalkylammonium hydroxide to be used as a developer in fine fabrication of circuit pattern and the like by photolithography in the manufacturing processes of integrated circuits such as LSIs or ultra LSIs or liquid crystal display units.
Semiconductor integrated circuits such as ICs, LSIs, ultra LSIs, etc. are produced as follows: Photo resist is coated on a substrate such as silicon wafer, and a desired pattern is put on it by exposure using a stepper. Then, photolithography process such as developing, etching, etc. are repeated. Thin film transistor (TFT) for liquid crystal display unit is also manufactured by photolithography process on a glass substrate.
Photo resist is divided into a negative type resist, in which irradiated portion is insolubilized by exposure of circuit pattern, and a positive type resist, in which irradiated portion is solubilized.
As a typical positive type photo resist, novolak resin containing photosensitive substance such as o-diazonaphthoquinone is used. An example of a mixture of esterized o-diazonaphthoquinone sulfonic acid ester and m-cresol type novolak resin is given by the following chemical formula: ##STR1##
The o-diazonaphthoquinone sulfonyl group combined with novolak resin gives an effect to decrease solubility of novolak resin. When light such as ultraviolet ray is irradiated on o-diazonaphthoquinone, it is turned to ketene, and 3-indene-carboxylic acid is further genenerated under the presence of water: ##STR2##
As the result, o-diazonaphthoquinone not only ceases to give effect as a dissolution inhibitor for novolak resin, but the alkali solubility is increased because acid is generated. Therefore, when a positive type photo resist with an exposed circuit pattern is developed by alkali solution, the exposed portion is dissolved by alkali solution, and a positive image is formed.
As alkali developer for the positive type resist, alkali carbonate, aqueous ammonia, tetramethylammonium hydroxide, etc. are used. With the increase of the devices with higher integration such as LSI, there are strong demands on the decrease of impurities in chemical agents to be used in the manufacturing process. Above all, there is strict restriction on intermingling of metal ions in the semiconductor manufacturing process. As the result, the developers containing tetramethylammonium hydroxide, which is an alkali solution containing no metal ion as main component, is now widely used in the process of photolithography.
Aqueous solution of hydroxide of tetraalkylammonium such as tetramethylammonium hydroxide used as a developer for the positive type photo resist had been disposed in the past as waste solution. This waste solution contains organic substances such as novolak resin, which is a major component of photo resist, and hydroxide of tetraalkylammonium contains nitrogen, which is turned to eutrophic component in water and causes contamination and deterioration of water quality. Thus, this has been disposed through various treatment procedures. With the increase in the quantity of developer used for the manufacture, the quantity of waste solution to be disposed also increased, and there have been urgent demands on the utilization of waste solution and on the decrease of the waste solution.
The present inventors have proposed a method for obtaining high purity tetraalkylammonium hydroxide from a cathode chamber in the Japanese Patent Application No. 2-408052 by introducing the waste solution into an anode chamber of an electrolytic cell, which is divided by a perfluorinated cation exchange membrane. According to this method, organic substances of novolak resin type in the waste solution, which is introduced into an anode chamber of an electrolytic cell, do not adversely affect the cation exchange membrane or move into the cathode chamber. But, when electrolysis is continued, concentration of organic substances such as novolak resin in the anode chamber increases, and the anode solution with increased organic substance concentration must be disposed, and it is impossible to completely regenerate and utilize tetraalkylammonium hydroxide in the waste solution.
Further, as a method for suppressing the increase of concentration of organic substances such as alkali-soluble novolak resin, which is accumulated in anode chamber when electrolysis is continuously performed, the present inventors have proposed a method in Japanese Patent Laid-Open Publication 5-7889 (U.S. patent application Ser. No. 08/68277 U.S. Pat. No. 5,354,434), in which carbon dioxide is introduced to neutralize waste developer solution and, after alkali-soluble organic substances are dissolved in it, electrolysis is performed in an anode chamber of electrolytic cell partitioned by a perfluorinated cation exchange membrane to obtain high purity tetraalkylammonium hydroxide from cathode chamber.
According to this method, waste developer solution is sent from the waste solution tank 1 to a neutralizing process 2 as shown in FIG. 3, and after carbon dioxide is blown in for pH adjustment, precipitated alkali-soluble substances are removed in a filtration process 3. Then, water component is separated in a condensation process 4, and organic solvents or surface active agents are reduced in an organic substance removal process 5, and it is sent to an anode chamber 8 of the electrolytic cell 6 partitioned by cation exchange membrane 7. Then, aqueous solution of tetraalkylammonium hydroxide is collected from the cathode chamber 9, and carbon dioxide generated in the anode chamber is re-used in the neutralization process.
In this method, however, water-soluble low grade organic substances of ethyl cellosolve type to be used as solvent for photo resist remain in the waste developer solution after neutralization and microfiltration as they are introduced into the anode chamber of the electrolytic cell. As the result, low grade substances increase in the anode solution as electrolysis is continued, and a part of them are dialyzed through the cation exchange membrane during electrolysis and is contained in tetraalkylammonium hydroxide collected in the cathode chamber. COD (Mn) value reaches as high as about 1000 ppm, and this is far higher than the level of less than 10 ppm in high purity tetraalkylammonium hydroxide commercially available as developer, and this is not desirable in some applications.
Under such circumstances, it has been proposed to design the electrolytic cell in multiple stages, or the solution is collected from the cathode chamber through at least two cation exchange membranes using a multi-chamber electrolytic cell. By this method, however, low grade organic substances migrate toward the cathode chamber as electrolysis is continued. To raise removal ratio of the low grade organic substances, a method has been proposed in Japanese Patent Laid-Open Publication 5-106074 (U.S. patent application Ser. No. 08/168277), in which ozone prepared by an ozone generator is supplied to aqueous solution of tetraalkylammonium hydroxide containing low grade organic substances, which passed through the first cation exchange membrane, in order to oxidize and decompose and to reduce content of low grade organic substances, and high purity tetraalkylammonium hydroxide with lower content of low grade organic substances can be obtained. Although removal ratio of organic substances contained as impurities is high, it is necessary to provide an ozone generator, an additional means for promoting reaction with ozone, an apparatus for decomposing unreacted ozone, etc. Also, alkylamine odor often occurs in tetraalkylammonium hydroxide obtained from the cathode chamber when the solution in an intermediate chamber of the multi-chamber electrolytic cell is continuously oxidized by high concentration ozone for long period. This may be attributable to the fact that tetraalkylammonium hydroxide is slightly decomposed and alkylamine is generated. The presence of alkylamine can be identified by odor even when it is present in such small quantity that it cannot be detected by an analyzer and it gives no effect on tetraalkylammonium hydroxide. Thus, there have been strong demands on tetraalkylammonium hydroxide which does not generate alkylamine.
It is an object of the present invention to provide a method for regenerating tetraalkylammonium hydroxide, which contains very low content of low grade organic substances as impurities, whereby there is no need to provide an ozone generator, an apparatus for promoting reaction with the solution to be treated, an apparatus for decomposing unreacted ozone, etc. as required in oxidation and decomposition of low grade organic substances by ozone, and oxidation and decomposition can be adjusted in easy and simple manner.